Data processing is commonly used, e.g., in the field of administration and management of enterprises, cost counting, cost allocation and bookkeeping. In various countries, recommendations and legal regulations have been released for the exchange of electronic documents in order to make the electronic exchange of documents and the electronic document itself suitable for being acknowledged as “documents” in the legal sense in the same way as traditional paper documents are. Examples for such recommendations are VAT guidelines and regulations of the European Union (EU) and CEN-Report for elnvoicing.
In Germany, electronic documents, particularly electronic invoices are acknowledged as documents in a legal sense provided that the electronic document, i.e. the electronic invoice, is digitally signed, that the receiver of the document verifies the signature and that the sender and receiver archive the document. Further, the signature has to comply with the German signature law. This means that the signature has to be a qualified signature. Further requirements on electronic documents, particularly invoices, are based on the principles of orderly bookkeeping. Regulations for an acknowledged archiving in a legal sense comprise that the electronic document to be archived has a qualified signature in case of electronic invoices, that the electronic document is stored on an unchangeable storage medium, that converted and unconverted data are archived with respect to an index comprising a link to a converting mechanism if applicable, and that the archiving procedure is recorded.
Using digital signatures requires a public key infrastructure (PKI). The main body of such an infrastructure, is a Certification Agency (CA) that issues, stores and distributes digital certificates (“electronic passports”). To acquire a digital certificate, a public and a secrete (also entitled private) key pair has to be generated locally. A certificate request, containing only the public key is sent to the Registration Agency (RA) of the CA or directly to the CA. The CA has defined processes to check the request e.g. personal appearance with passport at the RA may be required, and to issue the certificate. The secrete key has to be kept under complete control of the certificate owner, because it is used to digitally sign or decrypt documents. The public key may be used by the public (e. g. business partner) to verify signatures or to encrypt private or confidential messages for the certificate owner. Examples of CA's are VeriSign Inc. (Mountain View, Calif., USA) or TC TrustCenter AG (Hamburg, Germany). CA's and RA's are also known as Trusted Third Parties (TTP) and they usually offer services to retrieve certificates, to check for certificate revocation and for time stamping documents.
The time of validity of certificates is usually limited. Additionally, certificates can also be revoked before the time limit is reached, for example when the underlying secrete key may no longer be secret. Therefore a digitally signed document may need to be re-signed or a new time stamp may be applied by a trusted third party with the respective actual newest encryption technology before the time limit of the last used certificate is reached. By properly time stamping the document the continued authenticity of the original signer and the integrity of the document can be ensured over any time period, also in case that an older key applied or an older encryption technology applied may be cracked in the meanwhile.
Software applications and systems which provide a solution to part of the regulations pointed out above are known. Software applications for sending and receiving digitally signed electronic invoices are also known. The sending module allows on a sender's computer system creating a digital signature for documents, creating an envelope comprising the document and the digital signature, and sending the envelope to a receiver module on a receiver's computer system. The term “signed document” is alternatively used for the envelope.
Creating a digital signature comprises calculating a hash value which is unique for the document to be sent. The result of encrypting the hash value using the secrete key is equivalent to the digital signature. For transmission the digital signature may be converted into an ASCII code string. Suitable signature processes are known in the prior art.
The receiver module allows on a receiver's computer system receiving an electronic document via a communication interface, decrypting the signature by means of a public key of the sender, validating the signature, archiving the document, the signature, the public key of the sender, the result of a converting process, if applicable, and a report of these various processes, in passing the document to a business software system of the receiver. Validating the signature may, for example, be implemented by a method comprising calculating a hash value of the document by means of the same process that has been applied on the sender side and comparing that calculated hash value with the hash value obtained by decrypting the signature received with the document using the public key of the signer. Identity of both values reveals the originality and integrity of the document.
A hash value is also known as “digest”. It is a number which is calculated for a document by a one-way function. The algorithm of the one-way function is highly sensitive with respect to changes in the document, and therefore the hash value is unambiguous for the document. However, it is not possible to retrieve information about the content of the document from the hash value. The algorithms for calculating hash values are public and not secret.
When an electronic document has to be transmitted confidentially it is encrypted with the public key before sending and transmitted in the encrypted form. It can be opened by the receiver only when he knows the secret key corresponding to the public key used upon encrypting the document. Because encryption and decryption of large documents using public key cryptography is expensive in terms of computing effort, public key cryptography is only used to exchange a symmetric secret key which is then used, to actually encrypt and decrypt the document using symmetric algorithms such as DES (Data Encrypting Standard of NBS).
In the context of this disclosure, a proof of authenticity is understood to be the proof that an electronic document comes from a specific person or legal entity defined in the digital certificate.
The security of the contemporarily used encrypting methods relies on the fact that it is presently not possible to reduce large numbers into prime factors with a fast algorithm. Even very fast programs require some thousand years of calculation time to reduce a number with two hundred digits into prime factors. However, in case that this could be done faster one day, the cryptosystem can be cracked.
For certain electronic documents, particularly for invoices under certain legal regulations, a proof of authenticity over many years is required. Examples for such electronic documents are contracts and invoices or generally, like in Germany, documents where a “qualified” digital signature is legally required.
Because the methods and algorithms for proving authenticity of electronic documents used at the time an electronic document is created and digitally signed, may become insecure (e.g., the method used may be cracked due to the development of the corresponding techniques) secure methods and algorithms must be reapplied periodically by re-signing or time stamping the electronic document and its digital signature. In the context of a legally qualified signature of an electronic documents it is prescribed to re-sign the electronic signature of documents in given time distances, wherein upon each re-signing a presently new encryption method can be used and therefore cracking of the signature, the key or the electronic document can be prevented.
However, fulfilling these requirements or legal requirements for time stamping an electronic document having a digital signature, requires a search for the documents to be time stamped in large archives, retrieving the documents with their digital signature, performing the time stamping process and re-storing the time stamped documents in an archive. In a state of the art time stamping process therefore the document itself to be time stamped must be available in the time stamping process. This is not only inefficient and requires an expensive archiving technology, but also provides a risk in security, because the electronic documents have to be retrieved, handled and transmitted. Further, it is presently nearly impossible to entrust a service provider with performing the time stamping process, because the electronic documents would have to be transferred to the service provider, requiring a lot of effort in order to fulfill the secrecy and privacy requirements and in order to avoid spying of sensitive data.
For example, time stamping of electronic documents is of practical interest in “electronic bill presentment and payment (EBPP) systems”. Such systems provide for centralized and consolidated presenting and collecting of invoices for a sender or receiver of invoices. In such a system a service provider has for a certain time, typically three months or another accounting period, the required data and invoices for presenting the invoices and managing the payment. At the end of the accounting period the invoices are deleted in the system of the service provider for security reasons. For archiving purposes the service provider usually can store the data and invoices handled by his system on a data carrier, usually a CD-ROM, which is given to the customer of the EBPP system for archiving purposes.
The digital signature of the invoice provided by the service has to fulfill very high requirements, e.g. for allowing a pre-tax deduction. These legal requirements include time stamping within the validity of the last used signature certificate. Obviously, the effort involved in retrieving the documents for re-signing is very high. The EBPP system provider therefore would also like to offer long term archiving services, without the need of the original invoice data.
Thus, there is need for methods, software applications, data processing systems and computer programs providing a more efficient solution of at least a part of the problems described above. In particular, it is desirable to provide methods and systems for storing an electronic document having a digital signature in an improved manner allowing an easier long-term authenticity proof and further improved methods and systems for re-signing electronic documents having a digital signature.
The above description is based on the knowledge of the present inventor and not necessarily known in the prior art.